1. Field of the Invention
The present invention relates to multicolor image forming apparatus with two or more image forming units juxtaposed, such as plane paper copiers, laser beam printers, printing apparatus, and so on.
2. Related Background Art
The conventional multicolor image forming apparatus are generally so arranged that different color images are formed at a plurality of image forming units, a sheet of paper is conveyed by conveying means, for example such as a conveying belt, and the images are transferred one over another on the sheet to form a multicolor image. Particularly in the case where a full color image is obtained by development of multiple colors, even a slight overlap deviation will degrade the image. For example, in the case of 400 dpi, even an overlap deviation of a fraction of a pixel (63.5 .mu.m) will appear as changes in color and tone, so as to extremely degrade the image.
The conventional technology employed the way of relieving the overlap deviation of images by performing the multicolor development with a single image forming unit, that is, with a same scanning lens system, i.e., performing optical scanning under same optical characteristics. This method, however, had a problem that a considerable time is needed in outputting a multicolor image or full color image.
There is a method for solving this problem, in which separate optical scanning apparatus form images to obtain images of respective colors independently of each other and the images of respective colors are superimposed one over another on a sheet of paper conveyed by a conveying unit. A concern in this method, however, is a color deviation in superimposing the images.
A method effective against this color deviation is one as disclosed in Japanese Laid-open Patent Application No. 1-281468, in which marks for detecting a position of an image (image position detection marks), formed on a belt of the conveying unit, are detected and the image forming units are controlled to output the images of respective colors according to detection signals of the marks. Since the belt in the conveying unit generally moves up and down because of undulation etc., the detecting units of the marks for detection of image position need to detect accurate written positions even with up and down movement of the belt. Further, spectral reflectances of color toners etc. change depending upon colors in the case of light in the visible range. It is thus necessary to obtain stable outputs regardless of colors. Additionally, it is necessary for a light source in an illumination optical system for illuminating the marks for detection of image position to permit stable, high-quality detection of the marks.